1. Field of the Invention
The field of the invention is a valve operating system for an internal combustion engine comprising a valve-operation driving force transmission means interposed between a valve operating camshaft and an engine valve carried in an engine body for opening and closing operation of the engine valve, a valve operation mode changeover mechanism provided in the valve-operation switching the opening and closing mode of the engine valve in accordance with the operating condition of the engine, and a wrapping connector type transmission system provided between the valve operating camshaft and a crankshaft.
2. Description of the Prior Art
Valve operating systems of such type are conventionally known, for example, from Japanese Patent application Laid-open No. 134013/89.
In such a known valve operating system, it is common to operate the valve operation mode changeover mechanism, so that when the engine is in a higher speed operation, the lift amount of the engine valve is larger, and when the engine in a lower speed operation, the lift amount is smaller. In the higher speed operation, the load (shown by a solid line in FIG. 7) applied to a transmitting band when the engine valve is brought into the higher speed operation mode and the load (shown by a broken line in FIG. 7) applied to the transmitting band when the engine valve is brought into the lower speed operation mode each indicate a peak value at substantially the same number N.sub.p of revolutions of engine, e.g., at 4,000 rpm with the same preset load of the transmitting band, as shown in FIG. 7, due to a primary factor that the acting force on and the reaction force from a valve spring biasing the engine valve in a closing direction in the higher speed operation are larger than those in the lower speed operation. Moreover, the peak value is larger in the higher speed operation mode than that in the lower speed operation mode.
In the prior art, the preset load of the transmitting band in the wrapping connector type transmission system is constant and hence, if the switching from the lower speed operation mode to the higher speed operation mode is determined at the above-mentioned number N.sub.p of revolutions of engine or less, the load applied on the transmitting band may be excessively large during operation of the engine valve in the higher speed operation mode. Therefore, in order to avoid the application of an excessive load to the transmitting band, the switching from the lower speed operation mode to the higher speed operation mode must be determined at a number of revolutions of engine larger than the above-mentioned number of revolutions of engine N.sub.p, resulting in a reduced freedom of the timing for switching the operation mode of the engine valve.
A relationship between the preset load of the transmitting band and the load applied to the transmitting band is as shown in FIG. 8. When the preset load is lower (when the tension of the transmitting band is reduced), the load is of a peak level on a lower side of the number of revolutions of engine, as shown by a solid line in FIG. 8, whereas when the preset load is higher (when the tension of the transmitting band is increased), the load is of a peak level on a higher side of the number of revolutions of engine, as shown by a dashed line. Moreover, the peak load value in the event of a higher preset load is smaller than that in the event of a lower preset load. It is believed that this is due to a resonance phenomenon of the transmitting band, and the magnitude of the peak load is attributable to the magnitude of the amplitude of a resonance wavelength.